Keith Vaz, chairman of the parliamentary home affairs select committee, is facing pressure to explain why he failed to declare an interest when intervening in an official investigation into the business dealings of a close friend.

Vaz, the MP for Leicester East, wrote to the Solicitors Regulation Authority alleging 'discriminatory conduct' in its investigation into Dean and Dean, a legal firm owned by Shahrokh Mireskandari. He did not disclose that he was a close associate of Mireskandari, the solicitor at the heart of a number of discrimination cases against the Metropolitan police, and had invited him to public and private functions including Labour party events.

Vince Cable, the Liberal Democrat deputy leader, said Vaz should make a public statement to clear up his role in the affair.

'It is quite unreasonable that an independent regulator should have been undermined in this way. I would hope that the chairman of the home affairs select committee will give a full public statement,' he said.

The solicitors' authority launched an investigation into Dean and Dean in January after a number of complaints about its conduct. Vaz wrote to the authority's chief executive, Anthony Townsend, in February - on House of Commons notepaper - citing a complaint he had received from Mireskandari.

The authority was compelled to set up an independent working party to look into whether it had disproportionately targeted non-white lawyers for investigation.

The Observer has discovered that Vaz also introduced Mireskandari to members of the select committee when they travelled to Moscow in May and attended meetings with Russian ministers.

One member of the home affairs committee told The Observer that she was 'surprised and puzzled' when Vaz introduced Mireskandari to committee members when they visited Moscow on an official visit to discuss human trafficking.

Ann Cryer, the Labour MP for Keighley, said that she could not work out who he was and what he was doing there. 'There was this man there, a solicitor. He came to the functions that we went to but I could not work out who he was,' she said.

Another committee member said: 'He [Mireskandari] was very, very smooth with extremely well spoken English and appeared to speak very good Russian. I thought at the time that it was strange. He was a very interesting man. He even accompanied us to a meeting with Russian ministers in the Duma.'

This is not the first time Vaz has been asked to explain why he has made a serious allegation.

He was suspended from the House for a month in 2002 after a standards and privileges committee inquiry found that he had made false allegations against Eileen Eggington, a former policewoman.

The committee wrote at the time: 'We conclude that Mr Vaz recklessly made a damaging allegation against Miss Eggington to the Commissioner, which was not true, and which could have intimidated Miss Eggington or undermined her credibility.'

Repeated attempts were made to contact Vaz yesterday by telephone and email but he failed to respond. Last week, when the letter to the solicitors' authority was made public, he refused to comment.